3-(Hydroxyphenylphosphinyl)-propanoic acid, 3-HPP (I), which imparts flame retardant properties to polymers such as polyesters into which it is incorporated, has previously been prepared by reacting acrylic acid (II) and phenylphosphonous dichloride, PPD (III), in a condensation reaction, followed by the hydrolysis of the condensation mixture to give the product, according to the following scheme: ##STR1##
For example, Pudovik et al. [Russian Journal of Organic Chemistry, Vol. 37, pp. 423-427 (1967)] reported conducting the condensation reaction at temperatures up to 95.degree.-100.degree. C. and isolating the 3-(chlorophenylphosphinyl)-propionyl chloride (IV) as the condensation product in moderate (77%) yield by distillation. Subsequent hydrolysis of ##STR2## gave the desired product (I) in an overall yield of 71%.
An improved process was reported by Birum et al. in U.S. Pat. No. 4,081,463, the improvement being manifested as a greatly increased overall yield of at least 89.6% of (I) after hydrolysis. The basis of the Birum patent is the claim that the enhanced yield is achievable only when a very specific ratio of reactants is used in the condensation step, namely, a 25-45% molar excess, preferably 30-40% excess, of the acrylic acid (II) with respect to phenylphosphonous dichloride (III). It is further taught that if this critical and specific ratio of reactants is not utilized, as for example where equimolar quantities of reactants are employed, the yield of the desired 3-HPP (I) is unsatisfactorily low and, furthermore, that the product is impure and difficult to purify.
The stated explanation for these undesirable phenomena is that where only equimolar quantities of reactants are used the resulting condensation mixture still contains a substantial amount (ca. 25%) of unconverted PPD (III). By contrast, the condensation mixture resulting from the use of a large initial excess of acrylic acid (II) contains no residual PPD (III), but rather a mixture of three intermediates all of which are hydrolyzable to the desired product (I). These intermediates include not only the expected condensation product (IV), but also the corresponding cyclic anhydride (V) and the mixed anhydride (VI): ##STR3## Hence it is taught that it is critical to use the initial 25-45% molar excess of acrylic acid (II) to completely convert all of the PPD (III) and thereby generate the necessary mixture of intermediates (IV), (V) and (VI) to give the maximum yield of 3-HPP (I).